Gray clouds arched over Great Falls on a recent Friday, with a brilliant dome of blue sky visible underneath it.

It was a “chinook arch,” an unusual-looking cloud formation common to mountain areas created by warm chinook winds blowing off the Rocky Mountains 50 miles to the west.

“They are kind of a unique cloud feature for this area,” said Dave Bernhardt, a meteorologist for the National Weather Service in Great Falls.

The best way to visualize how the winds can form an arch of clouds in sky spanning many miles from the Rocky Mountains is to imagine throwing a pebble into a pool of water, Bernhardt says.

Like a pebble creates a wave in the water, waves of wind are created when it flows over the mountains.

“That’s what we’re seeing when we’re seeing the wind coming over the mountains,” Bernhardt said.

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A western edge of a Chinook arch cloud formation as seen from Flag Hill in Great Falls on Oct. 14.(Photo: TRIBUNE PHOTO/JULIA MOSS)

Here’s how the invisible path of the wind can be followed by watching the clouds it leaves in its wake:

Winds blow downslope off of the Rocky Mountain Front.

Then, wind takes a big vertical jump and continues to flow up and down in a wavelike fashion as it moves east.

This shows wind jumping vertically after traveling downslope on the mountains.(Photo: Courtsey of the National Weather Service)

And if there’s enough moisture in the air, high-level clouds begin to form along the path of the chinook wind, marking its path. Clouds will form when there is a vertical motion in the atmosphere, Bernhardt said.

That vertical jump forms the western or front edge of the arch.

Ripples from the wave continue as the wind moves upward and east of the mountains. The path of the invisible wind becomes visible in the clouds that begin to form an arch along the path of the wind.

“The location where that wave goes up east of the mountains is the area you will develop the clouds,” Bernhardt said. “Your chinook arch will be in that upward airfield and spread east by strong winds aloft.”

“At a certain point, that wave comes back down again, and that can be the eastern edge of the chinook arch,” he said.

Chinook arches can extend up to 100 miles east of the Rockies, but many times they are small, Bernhardt said.

The waves dampen out the farther away the wind gets from the Rockies.

A classic chinook arch was visible in Great Falls on Oct. 14.

The dome of blue sky under the arch was the location where the wind had leveled off, Bernhardt said.

“You’re essentially seeing the clear zone between the mountains and the back edge of the arch,” Bernhardt said of the blue area. “And it isn’t always clear. Sometimes there may be a thin overcast. Very thin, but you’ll see sunlight filter through it.”

Chinook arches are not uncommon in areas where downslope-type conditions exist, but they won’t develop without enough moisture in the atmosphere, he said.

Meteorologists typically see them developing via satellite, then watch to see how they erode or build.

“This could happen almost anywhere in the world where there’s a mountain chain and the winds are coming over the mountains,” Bernhardt said. “They may be called different things in different parts of the world.”

There is no fixed time how long they last, with some lasting longer than others.

“Whether people really take notice of it or not, I don’t know,” Bernhardt said.